wal_decoding: mergme: copy editing by Alvaro
wal_decoding: mergme: Replace leftover debug code with low severity debug log entry

If a streaming physical replica connects to the postmaster it was printing:

    WARNING:  connecting to

This looks like it was an unintended side-effect of some BDR debug code.
Corrected the oversight, made the message more useful and reduced the severity.
wal_decoding: mergme: Fix error message when archive_level = minimal but wal streaming enabled

Was:

    FATAL:  WAL streaming (max_wal_senders > 0) requires wal_level "archive" or "hot_standby"

Added mention of logical too.
wal_decoding: mergme: trivial comment/indentation cleanup
bdr: bdr_count: mergme: handle the case where a replication identifier is unknown
wal_decoding: replication_identifier: Fix replication_identifier to be a shared rel

It was always intended to be such.

Likely fix for a bug found by Craig
bdr: mergme: Adjust length in process_remote_action
bdr: mergme: fix typo in comment
bdr: mergme: get out of inner loop early if user cancels
bdr: mergme: add error check for unexpected results
bdr: mergme: add XXX comment about missing unexpected-result check
bdr: mergme: rename bdr_count_current_offset to MyCountOffsetIdx

looks nicer ...
bdr: mergme: unwrap literal string
bdr: mergme: use symbolic constant, not (off-by-one) hardcoded value
bdr: mergme: More CamelCase variable renaming
bdr: mergme: adjust some comments in bdr.c

Based on questions from Alvaro.
wal_decoding: mergme: Move to-be-renamed LogicalDecodingCheckpointData into .c

There's no need at all for it to be known outside the implementation.
wal_decoding: reorderbuffer: tiny comment improvements
A new isolationtester, with multi-server support

The spec file can now start with declarations like this:

    conninfo "cname1" "port=NNNN dbname=XYZ"
    conninfo "cname2" "port=MMMM dbname=PQR"

And sessions may include connection declarations like this:

    session "s1"
    connection "cname1"

Both are optional. If no connection information is explicitly specified,
then the earlier behaviour (use conninfo from commandline or fall back
to "dbname=postgres") is retained.

If connection information is specified, we open one "global" connection
per conninfo (for lock checking) and one connection per session. Setup
SQL is executed only on conns[0], as before (but perhaps it would help
to execute it on all until DDL replication is implemented?).

Includes the rudimentary tests from before.